Phase-shifted distributed feedback type semiconductor laser diode capable of improving wavelength chirping and external reflection return light characteristics and method for manufacturing the same

ABSTRACT

In a distributed feedback type semiconductor layer diode including a semiconductor substrate, an optical guide layer formed on the semiconductor substrate, a diffraction grating having a phase shift region being formed between the semiconductor substrate and the optical guide layer, and an active layer formed on the optical guide layer,  
     κ L+A·Δλ≧B    
     where κ is a coupling coefficient of the diffraction grating, L is a cavity length of the diode, Δλ is a detuning amount denoted by Δλ=λ g −λ where λ g  is a gain peak wavelength of the diode and λ is an oscillation wavelength of the diode, A is a constant from  0.04  nm −1  to  0.06  nm −1 , and B is a constant from  3.0  to  5.0.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a phase-shifted distributedfeedback (DFB) type semiconductor laser diode (DFB-LD) and itsmanufacturing method.

[0003] 2. Description of the Related Art

[0004] DFB-LDs have been used as light sources in high-speed,long-distance and large-capacity optical fiber communications. Indirectly-modulated DFB-LDs, whose output amplitude is modulated by apump circuit, the carrier density within an active layer and theequivalent refractive index fluctuate, which induces a spectrum spreadcalled a dynamic wavelength shift or a wavelength chirp.

[0005] In order to suppress the wavelength chirp, a prior artphase-shifted DFB-LD has been suggested (see: JP-A-2000-077774 andJP-A-2000-277851). That is, a λ/n phase shift (n>4, preferably, n=5˜8)where λ is an oscillation wavelength is located at a diffraction gratingof a waveguide.

[0006] Generally, in a DFB-LD, the fluctuation of a Bragg deviationamount Δβ is opposite in phase to the fluctuation of the optical output.In this case, note that a Bragg deviation amount Δβ is defined by

Δβ=2n _(eq)π(1/λ−1/λ_(B))

[0007] where n_(eq) is an equivalent refractive index;

[0008] λ is an oscillation wavelength; and

[0009] λ_(B) is a Bragg wavelength determined by the period of thediffraction grating, i.e., twice the period of the diffraction grating.

[0010] Also, assume that the phase shift value is less than λ/4, forexample, λ/5˜λ/8. In this case, the larger the Bragg deviation Δβ, thesmaller the mirror loss α_(m). Note that the Bragg deviation Δβ and themirror loss α_(m) determine an oscillation mode. Further, the smallerthe mirror loss α_(m), the larger the optical output. Therefore, whenthe optical output is increased by the external reflection return light,the Bragg deviation Δβ is decreased so that the mirror loss α_(m) isincreased, thus decreasing the optical output. Contrary to this, whenthe optical output is decreased by the external reflection return light,the Bragg deviation Δβ is increased so that the mirror loss α_(m) isdecreased, thus increasing the optical output. Therefore, a negativefeedback control by the external reflection return light is performedupon the optical output, so that the fluctuation of the optical outputcan be suppressed, which also suppresses the wavelength chirp.

[0011] Note that JP-A-2000-277851 provides a λ/5 to λ/8 phase-shiftedDFB-LD including a multiple quantum well (MQW) active layer formed by atensile-strained well layer, thus realizing the above-mentioned negativefeedback control.

[0012] In the above-described prior art phase-shifted DFB-LD, however,since the wavelength chirping characteristics and the transmissioncharacteristics strongly depend on parameters of the DFB-LD, thewavelength chirping and transmission characteristics cannot be improved.Note that the wavelength chirping characteristics dominates thetransmission characteristics.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide aphase-shifted DFB-LD capable of improving the wavelength chirping andtransmission characteristics.

[0014] Another object is to provide a method for manufacturing such aDFB-LD.

[0015] According to the present invention in a DFB-LD including asemiconductor substrate, an optical guide layer formed on thesemiconductor substrate, a diffraction grating having a phase shiftregion being formed between the semiconductor substrate and the opticalguide layer, and an active layer formed on the optical guide layer,

κL+A·Δλ≧B

[0016] where κ is a coupling coefficient of the diffraction grating, Lis a cavity length of the diode, Δ λ is a detuning amount denoted byΔ=λ_(g)−λ where λ_(g) is a gain peak wavelength of the diode and λ is anoscillation wavelength of the diode, A is a constant from 0.04 nm⁻¹ to0.06 nm⁻¹, and B is a constant from 3.0 to 5.0.

[0017] Also, in a method for manufacturing a phase-shifted DFB-LD, aplurality of samples of the phase-shifted DFB-LD having differentnormalized coupling coefficients κL and different detuning amounts Δλare formed. Next, power penalties of the samples connected to an opticalfiber are measured. Next, values of the normalized coupling coefficientsκL and the detuning amounts Δλ of the samples with the power penaltiesare plotted in a graph. Next, κL+A·Δλ=B is determined where A and B areconstants in order to divide the samples into first and seconds areas inthe graph, so that most of the samples belonging to the first area havepower penalties smaller than a definite value and most of the samplesbelonging to the second area have power penalties not smaller than thedefinite value. Finally, a new phase-shifted DFB-LD having a normalizedcoupling coefficient κL and a detuning amount Δλ satisfying κL+A·Δλ≧B isformed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be more clearly understood from thedescription set forth below, with reference to the accompanyingdrawings, wherein:

[0019]FIG. 1A is a timing diagram showing a directly modulated signalapplied to a phase-shafted DFB-LD;

[0020]FIG. 1B is a timing diagram showing an oscillation frequency ofthe phase-shifted DFB-LD of FIG. 1A;

[0021]FIG. 1C is a timing diagram showing the optical output power ofthe phase-shifted DFB-LD of FIG. 1A;

[0022]FIG. 1D is a timing diagram showing an optical output power of anoptical fiber receiving the optical output power of the phase-shiftedDFB-LD of FIG. 1A;

[0023]FIG. 2A is a timing diagram showing a directly modulated signalapplied to a phase-shafted DFB-LD;

[0024]FIG. 2B is a timing diagram showing an oscillation frequency ofthe phase-shifted DFB-LD of FIG. 2A;

[0025]FIG. 2C is a timing diagram showing the optical output power ofthe phase-shifted DFB-LD of FIG. 2A;

[0026]FIG. 2D is a timing diagram showing an optical output power of anoptical fiber receiving the optical output power of the phase-shiftedDFB-LD of FIG. 2A;

[0027]FIG. 3 is a graph showing detuning amount-to-line widthenhancement factor characteristics of a phase-shafted DFB-LD;

[0028]FIG. 4 is a flowchart for explaining an embodiment of the methodfor manufacturing a phase-shifted DFB-LD according to the presentinvention;

[0029]FIG. 5A is a graph showing the Δλ to κL characteristics of thesamples with their power penalties at step 403 of FIG. 4;

[0030]FIG. 5B is a graph showing the straight line κL+A·Δλ=B at step 404of FIG. 4;

[0031]FIG. 5C is a graph showing the Δλ to κL characteristics of the newDFB-LD at step 405 of FIG. 4;

[0032]FIG. 6 is a partly-cut perspective view illustrating a firstexample of phase-shifter DFB-LD to which the method of FIG. 4 isapplied;

[0033]FIG. 7 is a graph showing the Δλ to κL characteristics of thesamples of the first example of FIG. 6;

[0034]FIG. 8 is a graph showing the Δλ to κL characteristics of samplesof a second example of phase-shifted DFB-LD to which the method of FIG.4 is applied;

[0035]FIG. 9 is a partly-cut perspective view illustrating a thirdexample of phase-shifter DFB-LD to which the method of FIG. 4 isapplied;

[0036]FIG. 10 is a graph showing the Δλ to κL characteristics of thesamples of the third example of FIG. 9; and

[0037]FIG. 11 is a graph showing the Δλ to κL characteristics of samplesof a fourth example of phase-shifted DFB-LD to which the method of FIG.4 is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] First, the principle of the present invention will be explained.

[0039] In a phase-shifted DFB-LD, light reflection is distributed by adiffraction grating and a phase shift region is located at thediffraction grating. Therefore, a light intensity distribution is notuniform along the axis direction. As a result, since light is confinedaround the phase shift region, an electric field at the phase shiftregion is strong. Also, since carriers around the phase shift region areconsumed by the induced emission in accordance with the strong lightintensity at the phase shift region, the carrier density of the phaseshift region is relatively low, so as to increase the refractive indexthereof due to the plasma effect. This is called a spatial hole burning(SHB) phenomenon. Thus, when the phase-shift DFB-LD is directlymodulated, the refractive index of the phase shift region fluctuates sothat the phase shift amount is effectively changed.

[0040] Generally, if no consideration is given to the SHB phenomenon,when a directly modulated signal is switched from “0” to “1”, theoscillation wavelength is increased as carriers are injected into thephase shift region. This is called an oscillation wavelength red shiftdue to the carrier injection effect. On the other hand, since the SHBphenomenon increases the effective phase shift amount, the oscillationwavelength is decreased due to the counter effect to the carrierinjection effect. This is called an oscillation wave blue shift.

[0041] The relationship between a coupling coefficient κ and thetransmission characteristics will be explained next with reference toFIGS. 1A, 1B, 1C and 1D and FIGS. 2A, 2B, 2C and 2D. Note that thecoupling coefficient κ is defined by

κ=π·Δn/λ _(B)

[0042] where Δn is the difference in refractive index depending on theperiod of the diffraction grating. Note that a normalized couplingcoefficient κL where L is a cavity length may be also used instead ofthe coupling coefficient κ.

[0043] Assume that a phase-shifted DFB-LD has a relatively smallcoupling coefficient κ, i.e., a relatively small normalized couplingcoefficient κL. In this case, when a directly modulated signal as shownin FIG. 1A is applied to the phase-shifted DFB-LD, the oscillationfrequency of the phase-shifted DFB-LD is increased from f₀ to f₀′, andthen, is decreased as indicated by X (red shift) in FIG. 1B due to thecarrier injection effect. Finally, since the SHB effect is weak so thatthe effective phase shift amount is small, the oscillation frequency isbrought close to f₁. In this case, the optical output power of thephase-shifted DFB-LD exhibits a relaxation oscillation as shown in FIG.1C. As a result, the above-mentioned red shift X decreases the groupspeed of a transmission pulse traveling through an optical fiber tospread a pulse waveform, so that it will takes a long time for theoptical output power of the optical fiber to stabilize as shown in FIG.1D.

[0044] Next, assume that a phase-shifted DFB-LD has a relatively largecoupling coefficient κ, i.e., a relatively large normalized couplingcoefficient κL. In this case, when a directly modulated signal as shownin FIG. 2A is applied to the phase-shifted DFB-LD, the oscillationfrequency of the phase-shifted DFB-LD is increased from f₀ to f₀′, andthen, is decreased to f₁′ as indicated by X (red shift) in FIG. 2B dueto the carrier injection effect. Finally, since the SHB effect is strongso that the effective phase shift amount is large, the oscillationfrequency is increased to f₁ as indicated by Y(blue shift) in FIG. 2B.In this case, the optical output power of the phase-shifted DFB-LD alsoexhibits a relaxation oscillation as shown in FIG. 2C in the same way asin FIG. 1C. As a result, the above-mentioned red shift X is suppressedby the blue shift Y, which increases the group speed of a transmissionpulse traveling an optical fiber to compress a pulse waveform, so thatit will take a short time for the optical output power of the opticalfiber to stabilize as shown in FIG. 2D.

[0045] Thus, the larger the normalized coupling effect κL, the betterthe transmission characteristics.

[0046] The relationship between a detuning amount Δλ and thetransmission characteristics of a phase-shifted DFB-LD will be explainednext with reference to FIG. 3. In FIG. 3, a detuning amount Δλ isdefined by

Δλ=λ_(g)−λ

[0047] where λ_(g) is a gain peak wavelength. Also, a linewidthenhancement factor a is defined by

α=(dn/dN)/(dG/dN)

[0048] where G is a gain coefficient;

[0049] N is a density of injected carriers; and

[0050] n is a refractive index.

[0051] That is, the detuning amount Δλ depends on the linewidthenhancement factor α.

[0052] As stated above, when the phase-shifted DFB-LD is directlymodulated, the density of injected carriers fluctuates, so that theoscillation wavelength fluctuates due to the wavelength chirp. As aresult, in a long-distance transmission, when the wavelength chirp islarger, the transmission bandwidth is narrowed by the wavelengthdispersion. Therefore, the better the wavelength chirpingcharacteristics, the better the transmission characteristics.Incidentally, the wavelength chirping characteristics depends on thelinewidth enhancement factor α, which depends on the detuning amount Δλas shown in FIG. 3. Therefore, the wavelength chirping characteristicsdepends on the detuning amount Δλ.

[0053] Thus, the smaller the detuning amount Δλ, the better thetransmission characteristics.

[0054] In summary, the larger the normalized coupling coefficient κL andthe larger the detuning amount Δλ, the better the transmissioncharacteristics. The inventor found a special relationship between thetwo parameters, i.e., κL and Δλ for the better transmissioncharacteristics.

[0055] An embodiment of the method for manufacturing a phase-shiftedDFB-LD according to the present invention will be explained next withreference to FIG. 4.

[0056] First, at step 401, a plurality of samples of phase-shiftedDFB-LDs having different normalized coupling coefficients κL anddifferent detuning amount Δλ are formed.

[0057] Next, at step 402, each of the samples formed at step 401 isconnected to an optical fiber having a definite length for 100 -kmtransmission, for example. Then, the power penalty of each of thesamples is measured.

[0058] Next, at step 403, the values of (Δλ, κL) of the samples areplotted in accordance with their power penalties as shown in FIG. 5A. InFIG. 5A,  indicates a sample having a power penalty smaller than adefinite value such as 1 dB and ◯ indicates a sample having a powerpenalty not smaller than the definite value 1 dB. That is, the sampleindicated by  has good transmission characteristics and the sampleindicated by ◯ has bad transmission characteristics.

[0059] Next, at step 404, a straight line κL+A·Δλ=B as shown in FIG. 5Bdividing an area formed by the samples indicated by  and an area formedby the samples indicated by ◯ is determined.

[0060] Finally, at step 405, a new phase-shifted DFB-LD satisfying thecondition κL+A·Δλ>B is formed. That is, the new phase-shifted DFB-LD hasa value (Δλ, κL) fell in a shaded area in FIG. 5C.

[0061] Thus, if the new phase-shifted DFB-LD just satisfies thecondition κL+A·Δλ≧B, the new phase-shifted DFB-LD exhibits goodtransmission characteristics.

[0062] Examples of values A and B of κL+A·Δλ=B will be explained next.

[0063] A first example is applied to a phase-shifted DFB-LD asillustrated in FIG. 6.

[0064] In FIG. 6, a diffraction grating 12 is formed on an n-type InPsubstrate 11 by an electron beam exposure and lithography process. Inthis case, the diffraction grating 12 includes regions 12 a and 12 bhaving the same period corresponding to a wavelength of λ and a λ/n(4<n<16) phase shift flat region 12 c having no diffraction gratingstructure therebetween. Also, formed on the diffraction grating 12 arean about 0.1 μm thick InGaAsP optical guide layer 13 a, a 0.7% morecompression-strained InGaAsP MQW active layer 14 including seven periodsof 6 nm thick well layers and six periods of 10 nm thick barrier layers,and an about 0.05 m thick InGaAsP optical guide layer 14 by a metalorganic vapor phase epitaxial (MOVPE) process or the like. The InGaAsPoptical guide layer 13 a, the InGaAsP MQW active layer 14 and theInGaAsP optical guide layer 13 b are mesa-etched to form a stripestructure which is sandwiched by an InP current block layer 15. Also, anabout 3 μm thick p-type InP clad layer 16 and an about 0.2μm thickGaInAsP cap layer 17 are formed thereon. Further, a p-type electrode 18a and an n-type electrode 18 b for injecting a current into the MQWactive layer 14 are formed on the InGaAsP cap layer 17 and the InPsubstrate 11, respectively, by a sputtering process. Finally, the deviceis cleaved to a waveguide length L, and anti-reflection (AR) coatinglayers 19 a and 19 b having a reflectivity of about 0.1% are applied tothe front side facet and the back side facet, respectively.

[0065] In one sample of the phase-shifted DFB-LD of FIG. 6, if theetching depth of the diffraction grating 12 is 0.013 μm, the couplingcoefficient κ is about 65 cm⁻¹. In this case, if the cavity length L is450 μm, the normalized coupling coefficient κL is 2.92. On the otherhand, the gain peak wavelength λ_(g) of the MQW active layer 14 is made1.58 μm, for example. In this case, if the period of the diffractiongrating 12 is 240.0 nm, the oscillation wavelength λ is 1.55 . Thus, thedetuning amount Δλ is 0.03 μm. In this state where (Δλ, κL)=(0.03 μm,2.92), when this sample was directly-modulated at 2.5 Gb/s and wassubject to a 100 km transmission, the power penalty thereof was smallerthan 1 dB. Other samples each having a value of κL from 1.8 to 3.0 and avalue of Δλ from 5 to 50 nm was directly-modulated at 2.5 Gb/s and wassubject to a 100 km transmission, the power penalties are shown in FIG.7. As a result, A=0.05 nm⁻¹ and B=3.0.

[0066] A second example is applied to a phase-shifted DFB-LD which isthe same as the first example of phase-shifted DFB-LD except that theInGaAsP MQW active layer 14 is modified to have a tensile strain of 7%or more. In this case, as shown in FIG. 8, A=0.05 nm⁻¹ and B=3.4.

[0067] A third example is applied to a phase-shifted DFB-LD asillustrated in FIG. 9.

[0068] In FIG. 9, a diffraction grating 32 is formed on an n-type InPsubstrate 31 by an electron beam exposure and lithography process. Inthis case, the diffraction grating 32 includes regions 32 a and 32 bhaving the same period corresponding to a wavelength of λ and a λ/n(4<n<16) phase shift flat region 32 c therebetween. Also, formed on thediffraction grating 32 are an about 0.1 μm thick AlGaInAs optical guidelayer 33 a, an 1.0% or more compression-strained AlGaInAs MQW activelayer 34 including seven periods of 6 nm thick well layers and sixperiods of 10 nm thick barrier layers, and an about 0.05 μm thickAlGaInAs optical guide layer 24 by an MOVPE process or the like. TheAlGaInAs optical guide layer 23 a, the AlGaInAs MQW active layer 24 andthe AIGaInAs optical guide layer 23 b are mesa-etched to form a stripestructure which is sandwiched by an InP current block layer 35. Also, anabout 3 μm thick p-type InP clad layer 36 and an about 0.2 μm thickGaInAsP cap layer 37 are formed thereon. Further, a p-type electrode 38a and an n-type electrode 38 b for injecting a current into the MQWactive layer 34 are formed on the InGaAsP cap layer 37 and the InPsubstrate 31, respectively, by a sputtering process. Finally, the deviceis cleaved to a waveguide length L, and AR coating layers 39 a and 39 bhaving a reflectivity of about 0.1% are applied to the front side facetand the back side facet, respectively.

[0069] In one sample of the phase-shifted DFB-LD of FIG. 9, if theetching depth of the diffraction grating 32 is 0.025 μm, the couplingcoefficient κ is about 55 cm⁻¹. In this case, if the cavity length L is450 μm, the normalized coupling coefficient κL is 2.47. On the otherhand, the gain peak wavelength λ_(g) of the MQW active layer 34 is made1.57 μm, for example. In this case, if the period of the diffractiongrating 32 is 240.0 nm, the oscillation wavelength λ is 1.55. Thus, thedetuning amount Δλ is 0.02 μm. In this state where (Δλ, κL)=(0.02 μm,2.47), when this sample was directly-modulated at 2.5 Gb/s and wassubject to a 100 km transmission, the power penalty thereof was smallerthan 1 dB. Other samples each having a value of κL from 1.8 to 3.0 and avalue of Δλ from 5 to 50 nm was directly-modulated at 2.5 Gb/s and wassubject to a 100 km transmission, the power penalties were measured. Asa result, as shown in FIG. 10, A=0.05 nm⁻¹ and B=3.0.

[0070] A fourth example is applied to a phase-shifted DFB-LD which isthe same as the third example of phase-shifted DFB-LD except that theInGaAsP MQW active layer 34 is modified to have a tensile strain of 1.0%or more. In this case, as shown in FIG. 11, A=0.05 nm⁻¹ and B=3.0.

[0071] In the above-described examples, since an active layer isconstructed by a compression-or tensile-strained MQW structure, A=0.05and B=3.4˜3.8. However, the present invention can be applied to otheractive layers constructed by bulks, quantum small lines or quantum dots.Therefore, if some allowance is given to the values of A and B, A ispreferably 0.04 to 0.06 nm⁻¹ and B is preferable 3.0 to 5.0.

[0072] As explained hereinabove, according to the present invention,since two parameters, i.e., a normalized coupling coefficient κL and adetuning amount Δλ are defined to satisfy that κL+A·Δλ≧B where 0.04nm⁻¹≦A≦0.06 nm⁻¹ and 3.0≦B≦5.0, the wavelength chirping and transmissioncharacteristics can be surely improved.

1.-8. (Canceled).
 9. A distributed feedback type semiconductor layerdiode comprising: a semiconductor substrate; an optical guide layerformed on said semiconductor substrate, a diffraction grating having aphase shift region being formed between said semiconductor substrate andsaid optical guide layer; and a 1.0% or more compression-strainedAlGaInAs multiple quantum well active layer formed on said AlGaInAsoptical guide layer, wherein κL+0.05·Δλ≧3.0 where κ is a couplingcoefficient of said diffraction grating, L is a cavity length of saiddiode, and Δλ is a detuning amount denoted by Δλ=λ_(g)−λ where λ_(g) isa gain peak wavelength of said diode and λ is an oscillation wavelengthof said diode.
 10. A distributed feedback type semiconductor layer diodecomprising: a semiconductor substrate; an optical guide layer formed onsaid semiconductor substrate, a diffraction grating having a phase shiftregion being formed between said semiconductor substrate and saidoptical guide layer; and a 1.0% or more tensile-strained AlGaInAsmultiple quantum well active layer formed on said AlGaInAs optical guidelayer, wherein κL+0.05·Δλ≧3.0 where κ is a coupling coefficient of saiddiffraction grating, L is a cavity length of said diode, and Δλ is adetuning amount denoted by Δλ=λ_(g)−λ where λ_(g) is a gain peakwavelength of said diode and λ is an oscillation wavelength of saiddiode. 11.-17. (Canceled).
 18. The distributed feedback typesemiconductor laser diode as set forth in claim 9, wherein said opticalguide layer comprises AlGaInAs.
 19. The distributed feedback typesemiconductor laser diode as set forth in claim 10, wherein said opticalguide layer comprises AlGaInAs.